Site-specific or targeted delivery of drugs is considered a valuable tool to improve the therapeutic efficacy and to reduce the toxicity of drugs. For example, antibody-drug-conjugates are known in the art, and consist of a recombinant antibody covalently bound to a small therapeutic compound (typically 300 to 1,000 Da) via a synthetic linker (S. C. Alley et al., Curr. Opin. Chem. Biol. 2010, 14:529-537).
Whereas non-targeted drug compounds or diagnostic compounds typically reach their intended target cells via whole-body distribution and passive diffusion or receptor-mediated uptake over the cell membrane, targeted drugs or targeted diagnostic compounds home-in and concentrate mainly at the targeted tissues. Consequently, targeted drugs (herein meant as drugs targeted by a targeting moiety) or diagnostic compounds require smaller dosages, while still allowing the drug to reach therapeutically or diagnostically effective levels inside or at the level of the target lesion or cells, thus improving the therapeutic or diagnostic window.
In this regard, it is important to note that, in general, it is favorable if the amounts of targeted therapeutic compounds that reach the target cells can be adjusted according to their efficacy; in other words, that the accumulation of therapeutic compounds at the target cell needs to be higher in case they are less potent (e.g., less cytotoxic). For diagnostic compounds, a high accumulation at the target cell is generally considered advantageous, unless this reduces the signal, such as, e.g., in the case of quenching.
The use of targeted diagnostic compounds is of great value for whole-body imaging and can be used for patient selection and response prediction for targeted therapeutics (personalized medicine) and for validation of therapeutic responses to targeted therapeutic compounds.
Furthermore, the preferred lipophilic or amphiphilic character of non-targeted drugs, which facilitates their easy passage over the cell membrane and which feature is not always in agreement with other requirements of the drug, is less relevant to targeted drugs. The targeting of therapeutic compounds or diagnostic compounds to specific cells is, therefore, a conceptually attractive method to enhance specificity, to decrease systemic toxicity and to allow for the therapeutic or in vivo diagnostic use of compounds that are, in principle, less suitable or unsuitable as systemic drugs. In general, drug delivery technologies are aimed at altering the interaction of the drug with the in vivo environment and achieve that objective by conjugation of the drug with other molecules, entrapment of the drug within matrices or particles, or simply by co-administration with other agents. The net result is either drug targeting or enhanced drug transport across biological barriers such that its bioavailability is improved with a reduction of the incidence of clinical side effects in subjects. Drug targeting is achieved when an alteration in the drug's biodistribution favors drug accumulation at the desired site, which site is usually remote from the administration site. Cell-selective delivery of therapeutic agents (drugs) can, in principle, be obtained by coupling drug molecules to targeting moieties, which targeting moieties are a member of a specific binding pair, i.e., a member from a pair of molecules, wherein one of the pair of molecules has an area on its surface or a cavity that specifically binds to, and is, therefore, defined as complementary with, a particular spatial and polar organization of the other molecule, so that the pair have the property of binding specifically to each other. Examples of types of specific binding pairs are antigen-antibody, biotin-avidin, hormone-hormone receptor, receptor-ligand, enzyme-substrate, and IgG-protein A. Particularly suitable targeting moieties from such binding pairs are macromolecular carriers, such as, for example, monoclonal antibodies, antibody fragments or engineered variants thereof, or low molecular weight carriers, such as, for example, peptides.
However, the linkage between the therapeutic compound and the targeting moieties often poses significant problems. For instance, the linking of lipophilic non-targeted therapeutic compounds to hydrophilic targeting moieties may be difficult, although methods are available in the prior art to achieve such linking. Furthermore, chemically reactive groups for conventional conjugation chemistry may be absent, or chemically reactive groups may be (abundantly) present, but (covalent) linkage may (irreversibly) inhibit the bioactivity of the coupled therapeutic compound.
As will be explained in more detail below, with the method of the disclosure, it is possible to prepare conjugates wherein the bioactivity of such compounds is substantially maintained or even improved, as may be observed by an increased half-life of such compounds.
In WO2007/011217, a method is described for linking therapeutic compounds to targeting moieties by using (transition) metal ion complexes. These metal ion complexes have a first reactive group for forming a coordination bond with a functional moiety, such as a therapeutic compound or a diagnostic compound (such as a tracer) or a chelating agent and a second reactive group for forming a coordination bond with a site of the targeting moiety. In WO2007/011217, it is stated that typical reaction conditions for formation of the coordination bond between the second reactive group of the metal ion complex and the targeting moiety are 37° C. for twenty-four hours. These conditions are not compatible with batch-size production of compounds for clinical use. First, the reaction conditions are relatively harsh, which has a negative effect on the binding affinity (such as, for instance, the immunoreactivity) of the targeting moiety and interferes with approval for clinical use. Further, the conditions used ask for extreme sterility precautions during production to prevent bacterial contamination and endotoxin formation. Furthermore, the conditions used constitute an endpoint reaction that does not provide for the choice of an optimal ratio of drug/targeting moieties, depending on the type of drug or tracer to be coupled.
These risks and issues may at least be partially taken care of by the methods according to this disclosure.